What are the best algorithms for image segmentation tasks?
U-Net and U-Net inspired architectures have been quite popular in the medical image-related tasks ever since it was first introduced. There have been several improved versions of U-Net designed for specific tasks that followed. One such example is Attention U-Net, extremely popular for Pancreas Segmentation.

Other examples of architectures that have achieved state-of-the-art results in image segmentation tasks in recent years include Multi-Scale 3DCNN + CRF, popular for Brain and Lesion images, Multi-Scale Attention for MRIs, etc. A recent paper that describes an interesting 3D FCNN architecture is HyperDense-Net, widely used for multi-modal tasks in medical image segmentation.